1. Field of the Invention
The present invention relates to a zoom lens system and a camera including the same.
2. Description of the Related Art
In recent years, a camera such as a video camera, a digital still camera, a broadcasting camera, a monitoring camera which use a solid-state image pickup element, or a silver-halide film camera has provided high function and has been small in overall size.
A shooting optical system used in the camera is demanded to be a zoom lens system that has short overall length of a lens, small size, a high zoom ratio, and high definition.
Further, the shooting optical system is demanded to be a retractable zoom lens system including a configuration for reducing intervals between the respective lens units in non-shooting state to intervals different from those in shooting state and reducing a thickness (length in an optical axis direction) of the entire camera.
As a zoom lens system meeting those demands, there is known a zoom lens system that includes, in order from an object side to an image side, first, second, and third lens units respectively having positive, negative, and positive refractive powers and in which the respective lens units move to perform zooming (Japanese Patent Application Laid-Open No. 2005-106925 (corresponding to U.S. Pat. No. 7,123,422)).
There is also known a positive-lead zoom lens system that includes, in order from an object side to an image side, a lens unit having positive refractive power, a lens unit having negative refractive power, and a rear unit including one or more lens units following those lens units (Japanese Patent Application Laid-Open No. 2005-338740 (corresponding to U.S. Pat. No. 7,283,310) and Japanese Patent Application Laid-Open No. 2007-171371 (corresponding to U.S. Pat. No. 7,319,562)).
Japanese Patent Application Laid-Open No. 2005-338740 (corresponding to U.S. Pat. No. 7,283,310) and Japanese Patent Application Laid-Open No. 2007-171371 (corresponding to U.S. Pat. No. 7,319,562) disclose a zoom lens system that includes four lens units respectively having positive, negative, positive, and positive refractive powers in the stated order from an object side to an image side, and in which the respective lens units move to perform zooming.
Japanese Patent Application Laid-Open No. 2007-338740 (corresponding to U.S. Pat. No. 7,319,562) discloses a zoom lens system that includes a second lens unit including two lenses, i.e., a negative lens and a positive lens, and appropriately sets refractive powers of the components to realize a reduction in size of the entire system.
In recent years, translucent ceramics is developed. A shooting optical system employing the translucent ceramics as an optical material is known. The translucent ceramics have a high refractive index compared with optical glass and are excellent in hardness and strength. There is known a camera that realizes a reduction in thickness of an entire lens system making use of this characteristic (Japanese Patent Application Laid-Open No. 2006-84887 (corresponding to U.S. Pat. No. 7,407,334)).
In Japanese Patent Application Laid-Open No. 2006-84887 (corresponding to U.S. Pat. No. 7,407,334), translucent ceramics is used as a material of a negative lens of a cemented lens, which is formed by bonding a positive lens and the negative lens, to reduce lens thickness and realize a reduction in the entire lens system.
In recent years, there is a strong demand that a zoom lens system used in a camera should have a high zoom ratio and should be small in the overall lens system.
In general, in order to reduce the size of a zoom lens system, it is sufficient to reduce the number of lenses while increasing refractive powers of lens units forming the zoom lens system.
However, in such a zoom lens system, lens thickness increases, an effect of reduction in the length of the lens system is insufficient, and it is difficult to correct various aberrations.
Therefore, in order to realize a high zoom ratio and a reduction in size of the entire lens system, it is important to appropriately set a zoom type, the refractive powers of the lens units, a lens configuration forming the lens units, and the like.
In general, optical glass has a characteristic that, as a refractive index increases, an Abbe number decreases and dispersion increases.
On the other hand, it is known that ceramics has a higher refractive index compared with optical glass having an Abbe number same as that of the ceramics.
It is advantageous for aberration correction and a reduction in size of an entire optical system to use the ceramics having such a characteristic as an optical material.
However, even if a lens made of the ceramics is simply used for a zoom lens system, it is difficult to obtain high optical performance over an entire zoom range while realizing a reduction in size of the entire system.
In particular, in order to realize a high zoom ratio while reducing an effective diameter of a first lens unit to realize a reduction in the entire system in the above-mentioned positive-lead zoom lens system, it is important to appropriately set a lens configuration in a second lens unit. For example, it is important to select an appropriate material for lenses forming the second lens unit and to form the second lens unit with as small a number of lenses as possible.
In the positive-lead zoom lens system, if a lens configuration of the second lens unit is inappropriate, it is extremely difficult to obtain high optical performance over the entire zoom range while realizing a reduction in size of the entire system.